Agree completely with Yellowdog here.
Look at the collectibles from the 50s / 60s - see alot of demand for it. Why? Because that set is at the age (40+) where they are wanting to reconnect with something great from their past, causing the demand.
Arcade kids, like myself, from the 80s are reaching 40+, or better put an age where they want to reconnect with something from the past (could be any age, but most likely over 40).
I like to think that in 5 - 10 years there will be more demand based on this natural desire (and if Disney keeps making retro films that helps too!). Most will be entering a phase where the mortgage is paid, kids are grown, they have more time for liesure, so they will pick whats nearest to their hearts, and some will end up here, looking for the same parts we are. Its already happened, but i think it will continue.
50s and 60s you didnt have as much product, didnt have as much to go back to - collectible wise (cars, vending machines, etc). That may justify the expensive auctions i see going on. 80s, you did - mass produced games. So there is more of it out there.
The decrease in parts is offset by an increase in community - as long as the community stays strong, great guys like Thisoldgame and Phoenix (and many others) will produce whats needed. For the rare titles, well, they are rare in the first place, and will be treated as such.
Price wise, i offer no speculation. Sure, demand may lead to inflated prices, artificial or not, but ultimately we are here for the fun of it, i look forward to others joining the hobby down the line.